3. Theoretical Modeling of the Chemical Nonequilibrium Flow Behind a Normal Shock Wave

The nonequilibrium flow behind a strong normal shock wave is a typical and classical topic for discussion. In theoretical research, Lighthill [1, 2] and Freeman [3], etc. have introduced the landmark ideal dissociating gas (IDG) model in 1950–1960s. By introducing the concept of the degree of dissociation in a dissociation-recombination reaction, they built the conservation equations and the state equation for a nonequilibrium dissociating gas, which greatly simplified the analysis and calculation on the nonequilibrium flow field behind a shock wave. Freeman also approximately analyzed the equilibrium degree of dissociation and characteristic nonequilibrium scale by using the semi-analytical semi-numerical method. Subsequently, based on this theoretical model, many other scholars [4‐7] have carried on further or more in-depth analyses on the related flow problems.